Rotor for an electric motor

ABSTRACT

The invention relates to a rotor for an electric motor, especially an electric line-start motor, comprising spaces ( 4  to  7 ) which receive permanent magnets ( 10  to  13 ) and extend in an axial direction, and spaces ( 20  to  25 ) that accommodate conductor rods and extend in an axial direction. In order for the rotor to run as regularly as possible, the spaces ( 20  to  25 ) accommodating the conductor rods are provided with a substantially elongate cross-section in at least one sector of the rotor while being embodied in a curved manner along the longitudinal axis thereof in said sector when viewed from a cross-sectional perspective.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in international PatentApplication No. PCT/DK2003/000862 filed on Dec. 12, 2003 and GermanPatent Application No. 102 61 763.5 filed on Dec. 19, 2002.

FIELD OF THE INVENTION

The invention concerns a rotor for an electric motor, particularly anelectric line-start motor.

BACKGROUND OF THE INVENTION

The term electric line-start motor is used for hybrid a.c. motors, whichrepresent a combination of an a.c. asynchronous motor with an a.c.synchronous motor. Such an electric line-start motor comprises a statorwith several stator windings. The stator windings generate a rotatingfield, which generates a voltage in a rotor, which causes the rotor torotate. The rotor of an electric line-start motor comprises features ofboth the rotor of an a.c. asynchronous motor and of the rotor of an a.c.synchronous motor. Line-start motors can also be dimensioned forone-phase mains supply, if required using an operating capacitor.

In the rotor of an a.c. asynchronous motor, which can also be calledinduction motor, conductor rods, for example of aluminium or copper, arelocated substantially in the axial direction. At the front sides of therotor, the conductor rods can be connected by short-circuit rings.Together with the short-circuit rings, the conductor rods form the rotorwinding and can have the shape of a cage, which is the reason why such arotor is also called a squirrel cage rotor. During operation, therotating field of the stator winding causes a current change in theconductor loops of the initially still standing rotor. The currentchange speed is proportional to the rotational speed of the rotatingfield. The induced voltage permits current to flow into the rotorconductor rods connected by short-circuit rings. The magnetic fieldgenerated by the rotor current causes a torque, which drives the rotorin the rotation direction of the stator rotating field. When the rotorwould reach the rotational speed of the stator rotating field, thecurrent change in the conductor loop concerned, and thus also the torquecausing the rotation, would be zero. Therefore, in a.c. asynchronousmotors, the rotor speed is always smaller than the rotating field speed.Thus, the speed of the rotor is not mechanically synchronous with therotating field speed.

In the rotor of an a.c. synchronous motor, for example, permanentmagnets can be located, which generate a magnetic rotor rotational fieldduring operation. When the stator winding is provided with alternatingcurrent, the poles of the rotor are attracted by the counter-poles ofthe stator rotating field and shortly after repulsed by its uniformpoles. Due to its mass inertia, the rotor cannot immediately follow thestator speed. When, however, the rotor has almost reached the speed ofthe stator rotating field, the rotor is, in a manner of speaking, pulledinto the stator rotating field speed and runs on at that speed. Thismeans that after the start of the rotor, the rotor runs synchronouslywith the stator rotating field speed.

The rotor of an electric line-start motor comprises both permanentmagnets and conductor rods. The conductor rods form a starting aid forthe rotor. When the speed of the stator rotating field has almost beenreached, the permanent magnets evolve their effect. Thus, the electricline-start motor combines the good starting properties of anasynchronous motor, that is, large starting torque, with the highefficiency of the synchronous motor. When starting the motor, theconductor rods evolve their effect, whereas actually the permanentmagnets only have an interfering effect during the start of the motor.In synchronous operation, however, for example at 50 Hz or 3000 rpm, thepermanent magnets evolve their effect, whereas the conductor rods nolonger contribute to the generation of the torque, as no voltage isinduced into the conductor rods during synchronous operation.

The magnetic field existing in an air gap between the rotor and thestator during operation of the electric line-start motor comprises twocomponents. The first component of the resulting field is caused by thestator windings. This is also called rotating field. The secondcomponent of the resulting field is caused by the permanent magnets.During operation of traditional electric line-start motors, as knownfrom, for example, WO 01/06624 A1, torque fluctuations may occur, whichare not desired.

SUMMARY OF THE INVENTION

One object of the invention is to provide a rotor particularly for anelectric motor which makes the magnetic field approximately sine-shapedduring synchronous operation.

It is desired that during synchronous operation the field strength ofthe magnetic field existing between the rotor and the stator isapproximately sine-shaped. This, however, is counteracted by thepermanent magnets in the rotor, which cause an angular course withtraditional electric line-start motors. The desired sine-shaped rotatingfield is distorted by the traditional permanent magnets, thuscontributing, during the synchronous operation, to torque fluctuationsor torque pulsations, respectively. This undesired distortion duringsynchronous operation is generated in that the field strength of thepermanent magnet is distributed in an unweakened manner over the rotorsurface. In the direction of the magnet axis, the permanent magnetsmainly determine the field. With traditional electric line-start motors,the rotor is thus only partly completely conductive for the magnet fieldof the stator in the direction of the neutral axis, not, however, in thedirection of the magnet axis.

The rotor according to the invention is preferably a rotor for anelectric motor, particularly an electric line-start motor with axiallyextending receiving spaces for permanent magnets and with axiallyextending accommodating spaces for conductor rods. At least in a sectorof the rotor, the accommodating spaces for the conductor rods have asubstantially elongate cross-section. When regarded in thecross-section, the accommodating spaces for the conductor rods in thissector are curved along their longitudinal axis. During investigationsmade in connection with the present invention, it has turned out thatthe torque fluctuations occurring with traditional electric line-startmotors are caused by the fact that the course of the field strength ofthe resulting magnetic field in the air gap between stator and rotorover the rotation angle is not sine-shaped but, at least partly,angular. With the embodiment and the location of the accommodatingspaces for the conductor rods according to the invention, anapproximately sine-shaped course can be achieved during operation.

A preferred embodiment example of the rotor is characterised in thatseveral permanent magnets, particularly four permanent magnets, arelocated so that they generate a rotating magnet field with a neutralaxis and a magnet axis, which is arranged to be perpendicular to theneutral axis. The curvature radii of the accommodating spaces for theconductor rods decrease from the neutral axis in the direction of themagnet axis, which means that the curvature radii are smaller close tothe magnet axis than close to the neutral axis. The neutral axisextends, where the permanent magnets generate no magnet field. Themagnet axis extends, where the magnet field generated by the permanentmagnets is strongest. The field strength of the permanent magnet fieldcan, for example, amount to 1.5 Tesla. The magnet field generated by thestator windings runs from the stator through the rotor and back into thestator. The reduction of the curvature radii from the neutral axistowards the magnet axis causes that the magnet field lines occurringfrom the stator during the start can penetrate the rotor in a goodmanner.

A further preferred embodiment example of the rotor is characterised inthat the distance between the accommodating spaces for the conductorrods is constant. During tests made within the frames of the presentinvention, this embodiment has proved to be particularly advantageous.

A further preferred embodiment example of the rotor is characterised inthat in a cross-sectional view the accommodating spaces for theconductor rods are curved and arranged along their longitudinal axis insuch a manner that the distance of the accommodating spaces for theconductor rods to the rotational axis of the rotor, in a cross-sectionalview through the rotor, increases from the neutral axis in the directionof the magnet axis. This creates free spaces, in which the field linesof the magnet fields generated by the stator windings can penetrate therotor.

A further preferred embodiment example of the rotor is characterised inthat in a cross-sectional view through the rotor and disregarding thecurvature of the accommodating spaces, the longitudinal axes of theaccommodating spaces for the conductor rods are aligned substantiallyradially in relation to the rotor, and in that in a cross-sectional viewthrough the rotor the longitudinal axes of the accommodating spaces ofthe conductor rods are arranged to be turned in relation to the magnetaxis in such a manner that in a cross-sectional view through the rotorthe radial outer ends of the accommodating spaces for the conductor rodsare located at a smaller distance to the magnet axis than with a radialalignment. This means that in the vicinity of the magnet axis of thepermanent magnets the longitudinal axes of the accommodating spaces forthe conductor rods extend substantially in parallel to said magnet axis.This again causes that, also in the vicinity of the magnet axis; themagnet field generated by the permanent magnets can penetrate in anunhindered manner between the accommodating spaces for the conductorrods.

A further preferred embodiment example of the rotor is characterised inthat in a cross-sectional view each accommodating space for theconductor rods has two side walls, which have different curvatures.Thus, the accommodating spaces for the conductor rods have asubstantially sickle-shaped design.

A further preferred embodiment example of the rotor is characterised inthat the curvature radii of the side walls of the accommodating spacesfor the conductor rods are reduced from the neutral axis towards themagnet axis. The smaller the curvature radius of the side walls is, thesmaller is the length of the accommodating space enclosed by the sidewalls. During tests of the efficiency of the electric motor within theframes of the present invention, this has turned out to be advantageous.

A further preferred embodiment example of the rotor is characterised inthat in a cross-sectional view through the rotor, the inwardly turnedends of the side walls of the accommodating spaces for the conductorrods are connected by a rounded connecting wall. This has turned out tobe particularly advantageous from manufacturing-technical and functionalpoints of view.

A further preferred embodiment example of the rotor is characterised inthat the connecting walls of all accommodating spaces for the conductorrods have the same radius. This causes that radially inside the distancebetween the side walls is also constant.

A further preferred embodiment example of the rotor is characterised inthat the receiving spaces for the permanent magnets are curved andarranged around the rotational axis of the rotor in such a manner thatin a cross-sectional view through the rotor the distance between thereceiving spaces for the permanent magnets and the accommodating spacesfor the conductor rods is larger in the area of the magnet axis than inthe area of the neutral axis. This ensures sufficient space for themagnetic field lines of the magnet field generated by the stator.

A further preferred embodiment example of the rotor is characterised inthat in a cross-sectional view through the rotor the receiving spacesfor the permanent magnets have the shape of bows, which are arranged inthe shape of an ellipse, whose main axis covers the neutral axis andwhose auxiliary axis covers the magnet axis. During operation of thedevice according to the invention, this has turned out to beparticularly advantageous with regard to the distribution of the magnetfield lines.

A further preferred embodiment example of the rotor is characterised inthat the rotor has at least one transition zone, in which theaccommodating spaces for the conductor rods are not curved. The rotorcan have the shape of a rotor lamination mounted on a shaft. In thetransition zone laminated sheets may be arranged, which have no curvedaccommodating spaces for the conductor rods. The transition zone helpsachieving a so-called helical groove, meaning that a conductor rod at afirst end of the rotor is offset in relation to the conductor rod at theother end of the rotor. This offsetting, for example between 10 and 20mechanical degrees, is achieved in the transition zone, as the conductorrod does not run in parallel with the rotational axis of the rotor, butlaterally inclined. The helical groove causes a desired, substantialreduction of the amplitude of the magnetic harmonics interfering in therotary field. The transition zone comprises, for example, 10 to 20sheets, whose receiving spaces are offset in relation to each other.

A further preferred embodiment example of the rotor is characterised inthat the accommodating spaces for the conductor rods are closed on theradial outside. Preferably, the accommodating spaces for the conductorrods are located on the outer circumference of the rotor, and eventhough they have closed cross-sections, they can also be called grooves.The fact that the accommodating spaces are closed causes that thehigh-frequency shares in the magnet field induce no loss currents in theconductor rods.

With an electric motor, particularly an electrical line-start motor,with a stator comprising a plurality of windings, the task mentionedabove is solved in that a previously described rotor is arranged to berotational inside the stator. By means of the rotor according to theinvention, the magnet field during the start of the electric motor canbe controlled so that gaps in the magnet field of the permanent magnetscan be filled. With the approximately sine-shaped course of the magnetfield or the magnet field strength or the magnetic current intensity,respectively, efficiencies of more than 90% can be achieved.

A preferred embodiment example of the electric motor is characterised inthat short-circuit rings are arranged on the front sides of the rotor,said short-circuit rings connecting the conductor rods with each other.The short-circuit rings and the conductor rods form a cage, into which avoltage is induced.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention occur from thefollowing description, in which an embodiment example is described indetail with reference to the drawings, showing:

FIG. 1 is a view of a cross-section through the rotor;

FIG. 2 is a scaled down view of the rotor in FIG. 1 with field lines ofthe magnet field generated by the permanent magnets; and

FIG. 3 is the rotor in FIG. 2 with field lines of the magnet fieldgenerated by a stator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-section of a rotor 1 of an electric line-startmotor. The rotor 1 has a central through-hole 2 serving the adoption ofa shaft (not shown), via which the torque generated by the electricmotor can be supplied.

Four receiving spaces 4, 5, 6, 7 for permanent magnets 10, 11, 12, 13are located around the through hole 2. The receiving spaces 4 to 7extend in the axial direction, at least over a part of the length ofrotor 1. In a cross-sectional view, the four receiving spaces 4 to 7 forthe permanent magnets 10 to 13 are arranged in the form of an ellipse.The poles of the permanent magnets are marked by means of the capitalletters N for North Pole and S for South Pole. The arrangement of thepermanent magnets shown leads to the formation of a magnet field, whosefield strength is zero along a neutral axis 16 and largest along amagnet axis 17.

Towards the outside, the rotor 1 is bordered by a circular cylindershield, whose circumference accommodates a plurality of accommodatingspaces 20 to 25 and 28, 29 for conductor rods. The accommodating spacesfor conductor rods (not shown) extend in the axial direction over thewhole length of the rotor 1. In relation to the neutral axis 16 and themagnet axis 17, the rotor 1 in itself is made to be symmetrical. Thus,for reasons of clarity, only the accommodating spaces 20 to 25 and 28,29 for the conductor rods are provided with reference numbers.

Each accommodating space for a conductor rod, which could also be calledan accommodating space for a conductor winding, comprises two side walls31 and 32, which are connected by a semicircle-shaped connecting wall34. At the outer end, the longitudinal accommodating spaces for theconductor rods are made to be pointed or obtuse in a tapered manner. Thedistances 35 to 39 between the outwardly facing ends of theaccommodating spaces for the conductor rods are constant.

FIG. 1 shows that the two side walls of the accommodating space 28 aremade to be concave. As opposed to this, the two side walls of theaccommodating space 29 are made to be convex. The accommodating space 28is divided into two identical halves by the neutral axis 16 and theaccommodating space 29 is divided into two identical halves by themagnet axis 17. Each of the accommodating spaces 20 to 25, locatedbetween the accommodating spaces 28 and 29 and thus between the neutralaxis 16 and the magnet axis 17, has a convex and a concave side wall.The curve radius of the accommodating spaces 20 to 25 decreases from theneutral axis 16 in the direction of the magnet axis 17. This means thatthe accommodating space 20 has the largest curve radii and theaccommodating space 25 has the smallest curve radii.

FIG. 2 partly shows the magnet field generated by the permanent magnets10 to 13 in the form of magnet field lines.

FIG. 3 partly shows the magnet field generated by a stator (not shown)during the asynchronous start of the rotor in the form of magnet fieldlines. The arrows 48 and 49 in FIG. 3 show the magnetic current throughthe rotor 1.

The curved accommodating spaces for the conductor rods, which can alsobe called grooves, involve the advantage that the magnet field generatedduring operation of the electric line-start motor (not shown) is ledthrough the rotor 1 in a controlled manner. Thus, during operation ofthe electric motor, an approximately sine-shaped course of the fieldstrength of the resulting magnet field can be generated in the air gapbetween stator and rotor.

During synchronous operation of the electric motor the primary functionof the curve of the grooves or accommodating spaces, respectively, forthe conductor rods is to distribute the magnet field generated by thepermanent magnets in a sine-shaped manner in the air gap between rotorand stator. Accordingly, the magnet field is weakest in the area of theneutral axis and strongest in the area of the magnet axis.

Further, during the start of the electric motor, the curved design ofthe accommodating spaces for the conductor rods and the special locationof the conductor rods provide much space for the magnet field of thestator penetrating the rotor. As shown in FIG. 3, sufficient space forthe penetration of the magnet field lines is available between theaccommodating spaces 24 and 25 for the conductor rods and the permanentmagnets 11. Thus, magnetic bottlenecks are avoided, which could cause anundesired saturation of the rotor sheet. The special location of thepermanent magnets further increases the available space.

By means of the invention it is achieved that during the start of theelectric motor the magnet field is controlled so that gaps in the magnetfield, which are caused by the permanent magnets, are filled.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1. A rotor for an electric motor with axially extending receiving spacesfor permanent magnets and with axially extending accommodating spacesfor conductor rods, wherein in at least one sector of the rotor theaccommodating spaces for the conductor rods have a substantiallyelongate cross-section, and that in the at least one sector, in across-sectional view, the accommodating spaces for the conductor rodsare made to be curved along their longitudinal axis; and wherein in theat least one sector each accommodating space for conductor rods has twosidewalls, and, in a cross-sectional view, the two sidewalls of eachaccommodating space for conductor rods are curved in a similardirection.
 2. The rotor according to claim 1, wherein the distancebetween the accommodating spaces for the conductor rods in the at leastone sector is constant.
 3. The rotor according to claim 1, wherein aplurality of permanent magnets are located so that they generate arotating magnet field with a neutral axis and a magnet axis.
 4. Therotor according to claim 3, wherein in a cross-sectional view theaccommodating spaces for the conductor rods in the at least one sectorare curved and arranged along their longitudinal axis in such a mannerthat the distance from the accommodating spaces for the conductor rodsto the rotational axis of the rotor, in a cross-sectional view throughthe rotor, increases from the neutral axis in the direction of themagnet axis.
 5. The rotor according to claim 3, wherein in across-sectional view through the rotor, the curvature of theaccommodating spaces for conductor rods in the at least one sector issuch that a radial outer end of each accommodating space for conductorrods is turned toward the magnet axis, so as to be closer to the magnetaxis than if the accommodating spaces for conductor rods were not curvedalong their longitudinal axis.
 6. The rotor according to claim 3,wherein the receiving spaces for the permanent magnets are curved andarranged around the rotational axis of the rotor in such a manner thatin a cross-sectional view through the rotor the distance between thereceiving spaces for the permanent magnets and the accommodating spacesfor the conductor rods is larger in the area of the magnet axis than inthe area of the neutral axis.
 7. The rotor according to claim 6, whereinin a cross-sectional view through the rotor the receiving spaces for thepermanent magnets have the shape of bows, which are arranged in theshape of an ellipse, whose main axis covers the neutral axis and whoseauxiliary axis covers the magnet axis.
 8. The rotor according to claim1, wherein the rotor has at least one transition zone, in which theaccommodating spaces for the conductor rods are not curved.
 9. The rotoraccording to claim 1, wherein the accommodating spaces for the conductorrods are closed on the radial outside.
 10. An electric motor,particularly an electrical line-start motor, with a stator comprising aplurality of windings, wherein the rotor according to claim 1, isarranged to be rotational inside the stator.
 11. The electric motoraccording to claim 10, wherein short-circuit rings are arranged on thefront sides of the rotor, said short-circuit rings connecting theconductor rods with each other.
 12. A rotor for an electric motor withaxially extending receiving spaces for permanent magnets and withaxially extending accommodating spaces for conductor rods, wherein in atleast one sector of the rotor the accommodating spaces for the conductorrods have a substantially elongate cross-section, and that in the atleast one sector, in a cross-sectional view, the accommodating spacesfor the conductor rods are made to be curved along their longitudinalaxis; and wherein a plurality of permanent magnets are located so thatthey generate a magnet field with a neutral axis and a magnet axis, thecurvature radii of the accommodating spaces for the conductor rods inthe at least one sector decreasing from the neutral axis in thedirection of the magnet axis.
 13. A rotor for an electric motor withaxially extending receiving spaces for permanent magnets and withaxially extending accommodating spaces for conductor rods, wherein in atleast one sector of the rotor the accommodating spaces for the conductorrods have a substantially elongate cross-section, and that in the atleast one sector, in a cross-sectional view, the accommodating spacesfor the conductor rods are made to be curved along their longitudinalaxis; and wherein in a cross-sectional view each accommodating space forthe conductor rods in the at least one sector has two side walls, whichhave different curvatures.
 14. The rotor according to claim 13, whereina plurality of permanent magnets are located so that they generate arotating magnet field with a neutral axis and a magnet axis, and whereinthe curvature radii of the side walls of the accommodating spaces forthe conductor rods in the at least one sector are reduced from theneutral axis towards the magnet axis.
 15. The rotor according to claim13, wherein in a cross-sectional view through the rotor, the inwardlyturned ends of the side walls of the accommodating spaces for theconductor rods in the at least one sector are connected by a roundedconnecting wall.
 16. The rotor according to claim 15, wherein theconnecting walls of all accommodating spaces for the conductor rods inthe at least one sector have the same radius.
 17. An electric motorcomprising: a stator comprising a plurality of windings; and a rotorwith axially extending receiving spaces for permanent magnets and withaxially extending accommodating spaces for conductor rods, wherein in atleast one sector of the rotor the accommodating spaces for the conductorrods have a substantially elongate cross-section, and that in the atleast one sector, in a cross-sectional view, the accommodating spacesfor the conductor rods are made to be curved along their longitudinalaxis; and wherein the rotor is arranged to be rotational inside thestator; and wherein in the at least one sector each accommodating spacefor conductor rods has two sidewalls, and, in a cross-sectional view,the two sidewalls of each accommodating space for conductor rods arecurved in a similar direction.
 18. The electric motor according to claim17, wherein short-circuit rings are arranged on the front sides of therotor, said short-circuit rings connecting the conductor rods with eachother.